This invention relates to a current amplifier arrangement comprising
a current-current converter having an input connected to an input of the current amplifier arrangement, an output, and a power supply input to which a current source is connected, which current-current converter comprises an output transistor whose main current path is connected between the input and the output of the current-current converter and whose base is connected to the power supply input, and a diode-connected driver transistor connected between the base of the output transistor and a node carrying a reference potential during operation,
a current mirror circuit having an input connected to the output of the current-current converter and having an output connected to the output of the current amplifier arrangement.
Such a current amplifier arrangement is known, inter alia, from U.S. Pat. No. 4,803,441, in particular FIG. 6. The amplifier stage shown therein in fact comprises two such arrangements, one arrangement being connected to the positive supply voltage terminal for supplying a current of one polarity to the output and the other arrangement being connected to the negative supply voltage terminal for supplying a current of opposite polarity to the output. The combination of the two current-current converters has the advantage that the cross-over from the current of one polarity to the current of the other polarity at the output is effected in accordance with a linear characteristic.
Depending on the polarity of the input current, one of the current-current converters supplies a current to the associated current mirror circuit, which "reproduces" and outputs this current. If an accurate current mirror ratio is to be achieved for a large current range this will require the use of an accurate current mirror circuit. However, such accurate current mirror circuits are intricate and/or comprise so many transistors in series between the supply voltage terminal and the output of the amplifier arrangement that the voltage drop across them does not fit in the available supply voltage range. In such a case there will be no other choice than to revert to a current mirror circuit of a simpler type, which reduces the voltage drop across the current mirror circuit but often at the expense of the accuracy.
The inaccuracy of simple current mirror circuits is generally caused by the spread in transistor characteristics (gain, maximum current, substrate leakage current, and the like). It is known to reduce the effect of transistor spreads in such simple current mirror circuits by arranging resistors in series with the emitters of the transistors situated in the path between the supply voltage terminal and the output of the amplifier arrangement. These linearizing resistors reduce the influence of the base-emitter voltage of the transistors on the current mirror characteristics of the current mirror circuit. The values of the emitter resistors can be computed on the basis of the envisaged maximum current level through the current mirror circuit because the voltage drop across the current mirror circuit is maximal for this maximum current level. However, this means that the influence of the resistance thus computed is hardly noticeable at low current levels because the voltage drop across the resistance is then negligible relative to the base-emitter voltage. For smaller current levels such a simple current mirror circuit comprising linearizing resistors therefore has a poorer performance.
On account of the above problem it is difficult to achieve a fairly accurate current mirror ratio (unity or another constant ratio) over a wide current range by means of the known simple current mirror circuits. This applies in particular to current mirror circuits comprising PNP transistors.